The present invention is directed to a ball grid array (BGA) socket. More particularly the present invention is directed to a BGA socket that provides uniform loading of solder balls in a surface mount environment and method.
Electrical connectors/sockets may be used to secure electronic packages and/or integrated circuit (IC) devices, for example, onto a system board (e.g. a mother board or a printed circuit board xe2x80x9cPCBxe2x80x9d) of an electronic system. These electrical sockets may be constructed for easy installation and replacement of electronic packages (e.g. electrical components) and/or IC devices, such as complex memory chips and advanced microprocessor chips. The electrical sockets may also be available in different sizes and configurations, including, for example, low-insertion force (LIF) sockets and zero-insertion force (ZIF) sockets.
A BGA socket is commonly used for mounting central processing units (CPU) to PCBs. Contacts are assembled within the socket and flux is spread over the contacts. Solder balls may then be attached to each contact via the adhesive quality of the flux on each contact. With an array of solder balls in place, the socket/solder ball array is forced against a PCB whereupon a reflow process bonds the socket to the PCB.
In order to ensure reliable electrical connection between the solder balls on the socket contacts and associated conductive pads on the PCB, the balls must be loaded against the pads by a compressive force directed along an axis normal to the plane of the circuit board. Due to the co-planarity tolerances between the plane of the balls on the socket contacts and the plane of the circuit pads on the PCB, a significant force must be applied to the socket in order to ensure that a required minimum force is applied to each of the balls. Further, as the number of balls in the array increases, the total force which must be applied is increased.
In a perfect world all of the balls of an array should be pressed into mechanical and electrical contact with a circuit pad on the PCB with a uniform force. Ideally, all solder balls would have identical diameters. However, there may be significant variability in ball diameter, which means that the solder balls having slightly larger diameters than normal must be compressed through to a point where the slightly larger balls are of the same general height of the other, more nearly normal diameter, balls. The presence of significant variability in ball diameter height has plagued the industry. This variability has required the use of compressive force to secure the socket and ball array against a PCB, often resulting in solder ball cracking at a solder ball to terminal interface.
These cracked solder balls, when exposed to a reflow process where the solder balls are heated to a molten state and then allowed to solidify, frequently produce a poor electrically conductive path between the socket contact and circuit pad of the PCB.
One approach to minimize solder ball cracking at a socket contact to PCB circuit interface is to provide a socket contact that does not include a solder ball. These solder ball-free socket contacts are provided with simulated solder ball contact structures. These structures have their own inherent problems when the simulated solder ball contacts experience uneven loading due to a lack of co-planarity of the socket to the PCB.